1. Technical Field
Various embodiments relate to a semiconductor device, and more particularly, to a technology capable of preventing a malfunction during a refresh operation.
2. Related Art
Recently, more and more mobile electronic devices including smart phones need large-capacity DRAM. In general, data stored in a memory cell of a semiconductor memory device such as DRAM may be changed by leakage current. Thus, a refresh operation is required to periodically recharge the memory cell having data stored therein.
That is, a memory cell of a dynamic semiconductor memory such as DRAM, stores data in a capacitive element. Due to charge leakage from the capacitive element, the memory cell must be periodically refreshed. The refresh process typically includes a step of performing a read operation to acquire a charge level in the original state, the charge level being stored in the memory cell.
In particular, a semiconductor memory device including DDR SDRAM (Double Data Rate Synchronous DRAM) includes a plurality of memory banks for storing data, and each of the memory banks includes tens of millions of memory cells. Each of the memory cells includes a cell capacitor and a cell transistor, and the semiconductor memory device stores data through an operation of charging or discharging the cell capacitor.
Ideally, an electrical charge stored in the capacitor must always be constant when no specific control operation is performed. In reality, however, the electrical charge stored in the cell capacitor is changed due to a voltage difference from a surrounding circuit.
That is, electrical charge may leak in a state when the cell capacitor is charged, or electrical charge may be introduced in a state when the cell capacitor is discharged. When the electrical charge of the cell capacitor is changed, the cell capacitor may indicate that data stored in the cell capacitor is changed or lost. In order to prevent such a data loss, the semiconductor memory device performs a refresh operation.
Various types of refresh methods have been developed. According to an auto refresh method, a refresh timer exists outside a memory chip, and the memory chip performs a refresh operation in response to a periodic refresh command supplied by a controller.
According to a self refresh method, a refresh timer exists in a memory chip, and all memory chips request a refresh start command from a controller.